The method can preferably be used for the analysis and capture of cross-sectional images of an object, in particular a living object, that was captured by a magnetic-resonance tomography method. An apparatus according to the invention is preferably formed by a magnetic-resonance tomograph for spatially resolved nuclear magnetic-resonance measurement on a living object.
However, the invention is not limited to this concrete type of cross-sectional image capture but can basically be used for all techniques of cross-sectional image capturing that are technically available or still to be developed. Merely as examples can be mentioned: traditional x-ray tomography, computer tomography, positron emission tomography, electrical impedance tomography, neutron tomography, sonography etc. Moreover, the invention is not limited only to the medical field of use, but is preferably used there.
This method requires the existence of at least one cross-sectional picture that was produced using a cross-sectional imaging method, for example of the aforementioned type. In the meaning of the invention, a cross-sectional image is to be understood as a data set, in particular 2-dimensional data set of measurements, in particular intensity measurements. It is unimportant for the invention if this data set is actually illustrated as image. Preferably, the entire invention can work without illustration and only needs the data representing the cross-sectional image when carrying out the method.
The object and solution underlying the invention is explained in an exemplary and nonrestrictive manner by the technique of magnetic-resonance tomography (MRT).
Magnetic-resonance tomographs are generally known in the art and comprise substantially gradient coils for generating a plurality of in particular orthogonal magnetic fields, in particular in a Cartesian coordinate system, wherein usually a coil or coil arrangement is provided so as to generate a strong static magnetic field B0 along a Z-direction of a selected coordinate system, for example with field strengths of several Tesla. For this purpose, usually, superconducting coil arrangements are used.
Perpendicular and also parallel to the magnetic field direction B0 generated in this manner, moreover, further coils or coil arrangements are provided in order to generate magnet fields that are perpendicular to the static magnetic field and also at least one magnetic field that is parallel to this static magnetic field, wherein these magnetic fields are configured in particular as gradient magnetic fields, i.e., magnetic fields, the magnetic field strength of which changes over one of the coordinate axes. Through the interference it is achieved that the resonance frequency or the precession frequency of the spins changes depending on the spatially variable total magnetic field, whereby spatially resolved measurements can be implemented.
The measurement principle, in order to be able to achieve cross-sectional images in cross-section through living objects, is based on the fact that the spins, in particular hydrogen spins, of the living object initially align themselves within the static magnetic field B0 along the magnetic field direction and thus along the Z-axis.
By a high frequency excitation pulse that is adapted to the so-called Larmor frequency and is usually principally programmable with respect to the amplitude and the envelope, a deflection of the spins out of their equilibrium can take place so that the net magnetization Mz generated in the homogenous magnetic field B0 is deflected through the so-called flip angle so that a transverse magnetization component Mxy is present within the XY-plane of the selected coordinate system. Here, the flip angle depends substantially on the HF excitation pulse and is therefore programmable in an application-specific manner.
The transverse magnetization component Mxy generated in this manner is not temporally stable and due to different processes relaxes with different relaxation times, wherein the different processes superpose each other. These relaxations can be measured, in particular by receiving coils. The location of measurement is given by a volume element (voxel) of the object and is determined by a plurality of gradient magnetic fields that are at least temporarily superimposed on the static magnetic filed B0. Furthermore, it is known here to synchronize the start and/or preparation sequences of a measurement on a captured physiological motion.
These relaxation processes are known to the relevant person skilled in the art and are designated as T1-, T2- and T3-relaxation. Here, the T1-relaxation corresponds to the one at which the magnetization component Mxy flips back again in the direction of the Z-axis, whereas the T2-relaxation is based on a dephasing of the individual spins within the XY-plane and results in a weakening of the signal that is based on the emission of an electromagnetic wave due to the rotation of the transverse magnetization component in the XY-plane.
Furthermore, dephasings are superimposed on the T2-signal drop, which dephasings are given by macroscopic and microscopic magnetic field nonhomogeneities in the examined tissue or the examined object, which thus are based on differences in the magnetic susceptibility of the tissue. The effective relaxation rate, which includes T2-relaxation as well as susceptibility effects, is designated as T2* relaxation.
Besides the programming of at least temporarily acting gradient magnetic fields for achieving a spatial resolution in the examined object during the measuring preparation and/or signal reading, which is principally well known to the person skilled in the art, there are different possibilities to program so-called preparation and measuring sequences of pulses for controlling the respective coils (magnetic field coils and/or high frequency coils) so as to make T1-, T2- or T2*-relaxation times measured. In this context, this is also referred to as adequate weighting with respect to T1, T2 or T2* during the measurement acquisition.
With the possibility of spatially resolved measurements, 2-dimensional cross-sectional pictures through objects can be produced, wherein, correctly, such a picture represents at each pixel of the picture the signal from the respective volume element (voxel); thus, a cross-sectional image always represents not only an information of a mathematical plane, but also from a region extending in the layer thickness and perpendicular to the section. Such cross-sectional pictures, in turn, can be combined to form a 3-dimensional image.
The central challenges of magnetic-resonance (MR) imaging of the cardiovascular system include an unimpaired reproducibility, a spatial resolution in the millimeter range and in particular the imperative necessity of highly detailed geometrical imaging of the anatomy. In addition, a susceptibility-weighted MR presentation of the cardiovascular system requires imaging techniques that, with adequate effect, are able to reliably capture, present or quantify very small susceptibility-related differences between normal and abnormal tissue types in a diagnostically evaluatable manner.
High-quality MR illustration of moving organs requires the elimination of undesired influences due to breathing motion, heart motion, pulsation and blood flow. Therefore, the imaging has to be synchronized with the physiological motion or has to be adequately fast and thus immune with respect to the influences of motion.
Therefore, for an optimal illustration of moving organs, for example the freely movable heart, a compromise between short recording times and image quality has to be found. Short recording times can be implemented by a) ultrafast conventional imaging techniques, as b) parallel imaging as well as by c) accelerated imaging.
All these approaches for shortening the recording times can result in the improvement of insensitivity with respect to motion influences; however, at the same time, they cause significant deterioration in image quality and even nonusability of the images for diagnostic purposes.
The reason for this is speed increase-associated deterioration of the signal-to-noise ratio (SNR), the contrast-to-noise ratio (CNR) and also image interferences induced by fast, parallel and accelerated imaging, so-called artifacts, that can be introduced by the mentioned acceleration techniques. The development of the artifacts depends on the patients and the motion rate such as, e.g., heart frequency. In the image representation, these influences result in a widening of virtually real sharp boundary transitions between different areas of an object, in particular moving objects.
There are different approaches to implement quality control in the magnetic-resonance tomography. Test measurement with phantoms and test sequences allow to ensure the correct function of the magnetic-resonance tomograph and to assure constant measurement conditions. This test sequences that are to be repeated on a regular base check the function of the hardware.
Furthermore, in the clinical routine it is preferred to use only pulse sequences and protocols, the function of which has been sufficiently evaluated in advance. In order to evaluate the individual quality of a picture, there is the possibility to prospectively calculate the signal-to-noise ratio based on fundamental physical properties or to measure retrospectively. The signal-to-noise ratio reflects only a partial aspect of the perceived image quality and the exact determination of the signal-to-noise ratio is only possible with great difficulty.
Ultimately, up to now, the subjective experience of the physicians and user is necessary to evaluate the quality of the image data and thus decides if the images can be diagnostically used at a later time.